Cytometry and cell separation are very important in several fields like biochemistry, bioengineering and medicine. For example, in oncology, detection and analysis of even a single tumor cell is a fundamental step in diagnosis and treatment of different types of cancer. Several methods have been provided, for example optical analysis of a stained sample under a microscope (optical cytometry), attachment of cells to rare earth elements, ionization and cell separation (mass cytometry), etc.
These techniques require usually bulky and expensive equipment, and sometimes the sample preparation and analysis is lengthy and, in case of optical cytometry, not always reliable due to the limited amount of sample available to analyze.
Modern techniques include flow cytometers, which are able to analyze large amounts of particles in a relatively short time. Typically, the particles are labelled (for example, by staining) and included in a solution which is introduced in microfluidic channels, and lasers and/or fluorescence are used for studying the type and/or number of particles flowing in the solution. The main disadvantage of flow cytometry is potential overlapping of labels, making the analysis of the results less reliable and effective than, for example, mass cytometers.
Other types of cytometry use magnetic fields, for example magnetic twisting cytometry. In this technique, magnetic particles are attached to for example blood cells and viscoelastic effects are studied, for instance via optical means.
Magnetic force can also be used for cell separation. In “Cell manipulation with magnetic particles toward microfluidic cytometry” (Liu et al., J. Appl. Phys. 105, 102014 (2009)), magnetophoresis is proposed for cell separation in a microfluidic channel. The reference takes into account mixtures of target particles to be selected and non-target particles, both of which may present magnetic properties (for example, a solution of particles labelled with magnetic particles mixed with loose magnetic particles). The magnetic separation is performed with a gradient of a magnetic field, but this method either presents a low efficiency and reliability or results in problems like blocking of trap sites, aggregation and channel clogging, as well as heating problems.